@Leonard,
I'm supposing that you would be put into a very rudimentary iron age society. Of course, there are vestiges of civilization and what not, but the question is about what you would bring to the table once you got there. What would you do exactly to spur on the iron age society you suddenly became a part of
if you possessed advanced knowledge (either what you know right now at this very moment or what you could put down on a piece of paper).
My critique of the whole situation is this. People in the 21st century do not know (or do not remember) how to live as well as people probably a thousand years ago because they are so far removed from self sufficiency. What I mean is that with modern society comes a more developed network of professions and technology where the everyday person does not need to remember how to hunt for food, make a shelter, and so on. So say you got sick, you would assume that you could possibly go down to the emergency room, get a penicillin shot, and it would all be better. But what if you were transported back a thousand years ago and got sick with the exact same thing (keeping in mind that you know full well a cure for your ailment exists). Would you know how to prepare penicillin for your sickness? What would you be able to do right off the bat once you were transported back in time a thousand years ago which would bring you (and your village or whatever) closer to your specific time frame?
So say in my own case I were transported back right now without anything except my body and a conveniently placed, time traveling loincloth for modesty. What on earth could I do to make my life more like the life I have now. I only know a few things which would be somewhat more technologically advanced. I know philosophy and law which are somewhat practical, but what about tangible technology. Practically, I know engines. It's been a hobby of mine since I was very young, from internal combustion engines to steam and sterling engine designs. If I were transported back to the middle ages without anything except my time traveling loincloth, I would be able to introduce steam-driven engine technology to the dirty peons of the middle ages. Steam technology could help drive mills and so on, so it may be somewhat valuable. But I know that I would have problems with metallurgy and forging techniques required to make the metals able to withstand the stresses of steam driven technology. I would also probably have a pretty good grasp of agriculture, irrigation, and cultivation? at least more than the common peon.
That's where the second part of the question comes in. So say I were transported back in time with only my body, my time traveling loincloth, and a single piece of paper with any technology described on it. I would probably put down on the piece of paper techniques in forging, metallurgy, where to find the metals, and stuff like that. On the flip side of that paper, I would write down the mixture for gunpowder, how to make penicillin (seriously, the black death does not sound like a vitamin capable thing), probably how to make soap, and how to make very strong metals. I don't know, I think that being transported to the middle ages and surviving requires very strong metals. LOL!